Coenzymes

Working with the enzyme is frequently a co-factor, called a co-enzyme, a relatively small (compared with the enzyme) organic molecule which may itself be reversibly changed during the reaction. Remember that there are only five basic types of chemical reaction (see Figure 1.3) and all these types of reaction are encountered with enzymes and coenzymes.

, enzyme

TetL

, enzyme

FetT

enzyme

Figure 2.5 Schematic drawing of the centre of a haem group of a cytochrome enzyme catalysing an oxidation of an organic molecule. This is a free radical reaction, as indicated by the arrows with a single barb

2.2.1 Coenzyme A

Coenzyme A can be described as a "handle" for carboxylic acid groups. It picks up and drops acetyl groups, or acyl groups in general. It is particularly important in the degradation of fatty acids and in the first stages of terpene synthesis (Chapter 6). Its structure is shown in Figure 2.6. The essential part of the molecule is the thiol group, so it is usually represented as CoA-SH. The thiol reacts with, e.g., acetic acid, to give a thioester, coenzyme A thioacetate, or briefly CH3COS-CoA or AcS-CoA. Why a thioester? Esters are much more common in organic chemistry, but esters are less reactive than thioesters. Aldehydes and ketones have relatively reactive carbonyl groups, with reactivity slowed chiefly by bulky R groups. In esters, reactivity is decreased by the OR group through orbital overlap. There is no orbital overlap with the larger sulphur atom, so that reactivity of the thioester is more like that of a ketone. Notice the large increase in the acidity constant for the removal of an a-proton from a thioacetate compared with an acetate in Figure 2.7. The importance of this will become clear when considering the biosynthesis of fatty acids (Chapter 3) and terpenes (Chapter 6).